The present invention relates generally to a method for tuning nonlinear frequency mixing devices to yield continuous frequency tuning ranges across degeneracy.
Light sources are the heart of most modern optics systems. Specifically, tunable light sources exhibiting a wide wavelength range and high output stability are the very foundation of telecommunications networks, optical testing equipment (e.g., swept wavelength testing systems) as well as laser processing tools. Many of these applications require coherent light sources with wide, stable and continuous tuning.
Nonlinear frequency mixing devices are often used to generate light in certain wavelength ranges where suitable laser sources are not available (e.g., due to lack of lasing media generating light in those wavelength ranges at sufficient power levels). Nonlinear frequency mixing is also used for optical signal processing of data-containing signals (e.g., wavelength conversion, chirp reversal, temporal multiplexing and de-multiplexing). Nonlinear optics encompass various processes by which a nonlinear optical material exhibiting a certain nonlinear susceptibility converts input light at an input wavelength to output light at an output wavelength. Some well-known nonlinear processes involving light at two or more wavelengths (e.g., three-wave mixing and four-wave mixing) include second harmonic and higher harmonic generation, difference frequency generation, sum frequency generation and optical parametric generation. The fundamentals of nonlinear optical processes are discussed extensively in literature and the reader is referred to Amnon Yariv, Quantum Electronics, 2nd edition, Wiley Press, 1967 for general information.
The prior art teaches the use of nonlinear frequency mixers in signal processing. For example, M. H. Chou et al., xe2x80x9c1.5-xcexcm-band wavelength conversion based on difference-frequency generation in LiNbO3 waveguides with integrated coupling structuresxe2x80x9d, Optics Letters, Vol. 23, No. 13, Jul. 1, 1998 teach optical frequency mixing in the 1.5 xcexcm wavelength band for telecommunication purposes. Additional information related to nonlinear wavelength conversion for communications applications can be found in I. Brenner et al., xe2x80x9cCascaded "khgr"(2) wavelength converter in LiNbO3 waveguides with counter-propagating beamsxe2x80x9d, Electronics Letters, Vol. 35, No. 14, Jul. 8, 1999; and M. H. Chou et al., xe2x80x9cStability and bandwidth enhancement of difference frequency generation (DFG)-based wavelength conversion by pump detuningxe2x80x9d, Electronics Letters, Vol. 36., No. 12, Jun. 10, 1999. Though these devices were tunable, none of them operated through degeneracy.
The output light from nonlinear wavelength converters can be tuned over a certain wavelength range. In general, control of the wavelengths of the mixing or interacting input light beams can be used to adlust the output wavelength. When the nonlinear conversion process takes place in materials specially engineered to achieve high nonlinear conversion efficiencies, e.g., materials using quasi-phase-matching (QPM) gratings in in-diffused waveguides, control over certain grating parameters (i.e., the phasematching condition) can be employed to achieve output wavelength tuning. For general information on this subject the reader is referred to Michael L. Bortz""s Doctoral Dissertation entitled xe2x80x9cQuasi-Phasematched Optical Frequency Conversion in Lithium Niobate Waveguidesxe2x80x9d, Stanford University, 1995 as well as M. L. Bortz et al., xe2x80x9cIncreased Acceptance Bandwidth for Quasiphasematched Second Harmonic Generation in LiNbO3 Waveguidesxe2x80x9d, Electronics Letters, Vol. 30, Jan. 6, 1994, pp. 34-5. Additional information on devices using QPM gratings for nonlinear conversion is found in U.S. Pat. No. 5,875,053. The processes used to make QPM gratings are described in U.S. Pat. No""s. 5,800,767 and 6,013,221, and waveguides with QPM gratings employed for nonlinear optical processes are described in U.S. Pat. No. 5,838,720. Some specific high power pumped mid-IR wavelength systems using non-linear frequency mixing to obtain tunable light sources are taught by Sander et al, in U.S. Pat. No. 5,912,910.
The prior art also teaches the use of nonlinear frequency mixing in light sources. The use of optical parametric oscillators as tunable light sources is discussed by Mark A. Arbore et al. in xe2x80x9cSingly resonant optical parametric oscillation in periodically poled lithium niobate waveguidesxe2x80x9d, Optics Letters, Vol. 22, No. 3, Feb. 1, 1997. Also, the use of optical parametric oscillation for producing a tunable, short pulse and high repetition rate light source is taught by Kent Burr et al., xe2x80x9cHigh-repetition-rate femtosecond optical parametric oscillator based on periodically poled lithium niobatexe2x80x9d, Applied Physics Letters, Vol. 70, 1997, pg. 3343. The tuning bandwidth for the idler beam in Burr""s OPO extends from 1.68 xcexcm to 2.72 xcexcm and for the signal beam from 1.12 xcexcm to 1.50 xcexcm. However, the tuning of nonlinear conversion processes becomes problematic as one approaches degeneracy. Kent Burr et al. avoid degeneracy altogether in operating their OPO and hence do not generate any output in the wavelength range from 1.50 xcexcm to 1.68 xcexcm. In other words, they do not provide a light source with a continuous tuning range.
To better explain the problem of degenerate operation we will initially review a typical optical parametric oscillator (OPO) 1, as shown in FIG. 1. OPO 1 has a tunable laser source 2 for providing a pump beam 3 at a pump frequency xcfx89p. Pump beam 3 is in-coupled into a cavity 4 through an input coupler 5. Cavity 4 contains an optical parametric amplifier (OPA) 6 which receives pump beam 3 and produces in response a signal beam 7 and an idler beam 8. The output of OPO 1 is outcoupled from cavity 4 through output coupler 9. The output of OPO 1 typically includes at least one of the generated beams, i.e., signal beam 7 and/or idler beam 8. Optical parametric oscillation is supported by cavity 4 in OPA 6 and is a process during which pump beam 3 at pump frequency xcfx89p transfers power to signal beam 7 at frequency xcfx89S and to idler beam 8 at frequency xcfx89I according to the equation:
xcfx89p=xcfx89S+xcfx89I.
This process is performed such that energy and momentum are conserved between the photons of the three interacting beams, where in the case of quasi-phase-matching (QPM), momentum includes the k vector of the QPM grating. In the case where xcfx89S=xcfx89I=xcfx89p/2 the OPO is called degenerate and is essentially the opposite of second harmonic generation (SHG), such that:
xcfx89p=2xcfx89p/2.
In other words, degeneracy is encountered when frequency xcfx89S of signal beam 7 and frequency xcfx89I of idler beam 8 are equal to each other, and therefore equal to half of pump frequency xcfx89p of pump beam 3 which is driving OPO 1.
The approach to degeneracy and degeneracy itself are illustrated in FIGS. 2A and B. As seen in FIG. 2A, when pump beam 3 is tuned to a first pump frequency xcfx89p1 it establishes a gain spectrum A in OPA 6 with gain centered at a first signal frequency xcfx89S1 and at a first idler frequency xcfx89I1. Consequently, signal beam 7 and idler beam 8 will experience gain within their respective gain regions of spectrum A and typically contain a range of frequencies within those gain regions. Since the gain regions of spectrum A are far apart (non-overlapping), first signal frequency xcfx89S1 does not at any point overlap with first idler frequency xcfx89I1. It is therefore not possible for the same frequency to act as both signal and idler in this OPO.
As pump beam 3 is tuned to a second pump frequency xcfx89p2, a gain spectrum B is produced with gain centered at a second signal frequency xcfx89S2 and at a second idler frequency xcfx89I2. These two gain regions overlap a frequency range at 100. If OPO cavity 4 resonates frequencies in range 100, then an undesirable double resonance condition occurs in cavity 4 where both signal beam 7 and idler beam 8 resonate. Referring to FIG. 2B, while pump beam 3 is tuned to first pump frequency xcfx89p1 a bandwidth 101 is established, typically with the aid of a filter, within gain spectrum A for signal beam 7. The establishing of bandwidth 101 leads to the establishment of a corresponding mirror or image bandwidth 102 for idler beam 8 in accordance with the equation for OPO. Bandwidth 101 and mirror bandwidth 102 do not overlap. Hence, only signal beam 7 is resonant in cavity 4.
With pump beam 3 tuned to second pump frequency xcfx89p2 bandwidth 101xe2x80x2 and mirror bandwidth 102xe2x80x2 have an overlap 103. Now, signal beam 7 and idler beam 8 have a tendency to move into overlap 103, as indicated by the arrows and the double resonance condition occurs for signal beam 7 and idler beam 8. The double resonance condition becomes worse as pump beam 3 is tuned closer to a pump frequency xcfx89po and overlap 103 increases. At degeneracy gain spectrum C for both signal and idler beams 7, 8 is centered at the same wavelength, namely xcfx89po/2 (half the pump frequency) and bandwidth 101 completely overlaps image bandwidth 102. This is the point of mathematical degeneracy indicated in dashed and dotted lines D.
The instability of cavity 4 due to double resonance of signal and idler beams 7, 8 arising due to overlap 103 makes it highly undesirable to tune pump frequency xcfx89p and adjust bandwidth 101 in a range in which overlap 103 is significant. Consequently, pump beam 3 is only tuned to within a certain offset from degeneracy. Therefore, a certain range of frequencies for signal and idler beams 7, 8 is not available creating a wavelength xe2x80x9cdrop outxe2x80x9d in the tuning spectrum of OPO 1.
The issues associated with tuning through degeneracy also arise for other nonlinear frequency conversion mechanisms whenever portions of gain or signal or image spectra overlap. Thus, degeneracy causes similar tuning continuity problems in any device relying on nonlinear frequency conversion performed within and/or through degeneracy. As the prior art offers no solution to this problem, there is a need for a corresponding apparatus and technique to permit one to tune light sources using nonlinear frequency conversion elements near as well as through degeneracy.
It is therefore a primary object of the present invention to provide a light source using a nonlinear frequency conversion process which is tunable through degeneracy. Specifically, it is the object of the invention to provide a tunable light source which uses an optical parametric oscillator without suffering wavelength xe2x80x9cdrop outxe2x80x9d near and at degeneracy.
It is a second object of this invention to provide a widely tunable optical parametric amplifier or any other suitable nonlinear frequency converter without suffering wavelength xe2x80x9cdrop outxe2x80x9d near and at degeneracy.
This and other objects and advantages of the invention will become apparent upon further reading of the specification.
The objects and advantages are achieved by a method for tuning nonlinear optical frequency converters, including devices such as optical parametric amplifiers and optical parametric oscillators. In an optical parametric oscillator, which has a parametric amplifier placed inside a cavity, the tuning method involves providing a pump beam at an original pump wavelength to the optical parametric amplifier to produce a gain at a first wavelength and at a second wavelength. A passband is set around the first wavelength thereby generating a passband image around the second wavelength. During tuning, the original pump wavelength is adjusted to an adjusted pump wavelength when the passband is within a critical range of the passband image at said original pump wavelength. Typically, the critical range commences when the passband overlaps the passband image and the adjusted pump wavelength is selected such that the passband and the passband image do not overlap. Additional margins can be built into the critical range depending on the type of device used for setting the passband, the shape of the passband and the operating characteristics of the optical parametric oscillator.
In a preferred embodiment the passband is set by a narrowband tuner such as a diffraction grating filter, a tunable fiber Bragg grating or an etalon filter. These types of narrowband tuners are preferred because they are capable of producing a very narrow passbands as well as larger passbands, e.g., ranging from 0.1 pm up to 1000 pm.
Setting a passband in the cavity establishes a resonant wavelength. It is this resonant wavelength that is typically the output of the optical parametric oscillator. Now, when the passband is moved, the resonant wavelength is tuned. The moving can be performed continuously and it can be performed while the adjusted pump wavelength is held constant. The moving of the passband can also be performed as the adjusted pump wavelength is varied. Once the adjusted pump wavelength is set, the passband is moved such that the resonant wavelength corresponds to a point of degeneracy at the original pump wavelength. In this way, the resonant wavelength or the output of the optical parametric oscillator is tuned across the former point of degeneracy and thus avoids wavelength xe2x80x9cdrop outxe2x80x9d near and at the point of degeneracy.
The optical parametric amplification at the original pump wavelength is phasematched within a phasematching bandwidth. An offset between the original pump wavelength and the adjusted pump wavelength is chosen smaller than the phasematching bandwidth.
In the OPO embodiment the first and second wavelengths can be used interchangeably. For example, the first wavelength can be the signal wavelength and the second wavelength can be the idler wavelength. On the other hand, the first wavelength can be the idler wavelength and the second wavelength can be the signal wavelength. For example, the wavelength assignment will switch while tuning through degeneracy.
The optical parametric oscillator described above can be used in a tunable light source. The method of tuning the optical parametric oscillator ensures that the light source has a continuous tuning range through degeneracy without a wavelength xe2x80x9cdrop outxe2x80x9d. The light source has the cavity with the optical parametric amplifier positioned therein and a pump source for providing the pump beam. A spectral control mechanism is used for setting the passband around the first wavelength. As mentioned above, the spectral control can be a narrowband tuner selected among diffraction grating filters, tunable fiber Bragg gratings and etalon filters. A pump tuning mechanism is used to adjust the original pump wavelength to the adjusted pump wavelength. This adjustment is performed when operation at the original pump wavelength brings the passband within the critical range of the passband image.
In a preferred embodiment the tunable light source has a scan control for coordinating the setting of the passband with the adjustment of the original pump wavelength to the adjusted pump wavelength. Conveniently, the scan control is also used for tuning the resonant wavelength of the cavity before the switching to the adjusted pump wavelength.
The tunable light source is further equipped with a phasematching arrangement for phasematching the optical parametric generation taking place in the optical parametric amplifier. For example, the phasematching arrangement includes a quasi-phase-matching (QPM) grating.
The cavity is preferably chosen to avoid large mode-hops and other instabilities to ensure smooth tuning of the resonant wavelength. Preferably, the cavity is a multi-axial-mode cavity and is long. For example, the cavity can have a length of 1 meter or more. In some embodiments, the cavity can include an optical fiber. Conveniently, the cavity can be a ring cavity or a standing wave cavity.
The invention further provides for a method for tuning an optical parametric amplifier through degeneracy. In this case the pump beam is provided at the original pump wavelength to the optical parametric amplifier to produce gain at a first wavelength. Also, an input signal with an input spectrum at the first wavelength is provided. Together, the input spectrum and the original pump wavelength define an image spectrum at a second wavelength within the optical parametric amplifier. When the input spectrum and the image spectrum are within a critical range the original pump wavelength is adjusted to the adjusted pump wavelength. In this embodiment the critical range can be defined to commence when said input spectrum and said image spectrum overlap.
The method for tuning an optical frequency converter can be also practiced by providing to the frequency converter the pump beam to drive the optical frequency conversion. Also, at least one input signal with an input spectrum at a first wavelength is provided to the frequency converter. The input spectrum and the original pump wavelength are used to define an image spectrum at at least one second wavelength. When the input spectrum and image spectrum are within the critical range, the original pump wavelength is changed to the adjusted pump wavelength. In this embodiment the critical range can be defined to commence when the input spectrum and image spectrum overlap. Alternatively, a critical spectrum is defined around the first wavelength, thereby generating a critical spectrum image around the second wavelength. The original pump wavelength is then adjusted to the adjusted pump wavelength when the critical spectrum and critical spectrum image are within critical range.
As will be apparent to a person skilled in the art, the invention admits of a large number of embodiments and versions. The below detailed description and drawings serve to further elucidate the principles of the invention and some of its embodiments.